The 33-year-old Kazakh, who is fast building a reputation as one of boxing's most explosive and exciting fighters, knocked Monroe down three times in the bout before the referee stopped it 45 seconds into the sixth at The Forum in Los Angeles.
Golovkin improved to 33-0 and victory marked the 30th knockout in an impressive career which has seen him run roughshod over boxing's middleweight division.
Golovkin cracked Monroe with a right hand and dropped him in the second round. Then moments later Golovkin hit Monroe again with a sharp left for a second knockdown.
Monroe (now 19-2, 6 KOs) proved to be a willing combatant despite being hopelessly overmatched.
He tagged the champ with a combination of punches in the fourth that caused Golovkin to ask for more, but Monroe could not sustain any sort of consistent attack.
Golovkin came out looking for blood in the sixth. He popped Monroe again with a right uppercut and then backed the American into the ropes before dropping him for a third and final time.
Golovkin said after the fight that he wants to face Miguel Cotto or Canelo Alvarez next.